The invention relates to control of processors, and in particular to the provision of a standard interface for controlling multiple processors coupled together.
Control panels are used to provide an operator diagnostic capability to a processor. They permit a user to directly access processor registers and main storage in a processor, bypassing layers of processor software which could be faulty. Early control panels consisted of switches which were settable by an operator. Registers could be set by use of the switches, and their contents were represented by lights usually residing on a physical side or panel of the computer. More recent versions of control panels have included a service processor directly attached to a processor and its main storage. The service processor is then controlled by a typewriter keyboard, which may invoke programs in the service processor, to read and write register contents and main store data. Each processor is usually provided with its own control panel, if it has one at al. This results in much duplication of circuitry, since both I/O buses and the control panel have separate paths to main storage. Limited paths have also been provided to some registers through I/O buses.
U.S. Pat. No. 3,879,712 to Edge et al. describes a diagnostic interface attached to each processor. Each interface is also attached to a bus, and appears as a separate device from the processor it is attached to. The diagnostic interface is used by another processor to provide access to the processor the interface is attached to for running diagnostic routines. The interface provides access to some of the internal registers. One problem with this type of interface is that many I/O buses have a limited number of addresses available or a limited electrical load capacity. It severely limits the number of processors with diagnostic interfaces which could be coupled on an I/O bus. The interface provides only limited access to control registers, and thus does not provide a full control panel function. It is also separate from the I/O interface, and thus would require the above-mentioned duplication of circuitry.